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Solution to Molding Defects of Plastic Mold for Water Purifier Filter Shell

2026-04-30 11:27:59 Plastic Mold
The shell of water purifier filter is mostly injection molded from food-grade PP, ABS and PC plastic materials, belonging to thin-walled annular precision structural parts. It has strict requirements for surface flatness, dimensional accuracy and sealing compatibility, and must meet the sanitary standards for water contacting components. In actual mass production injection molding, affected by mold structure design, injection molding process parameters, raw material control and equipment status, various molding defects such as shrinkage depression, flash burr, bubble cavity and warping deformation often occur. These defects not only damage the appearance quality, but also cause assembly failure and sealing water leakage, greatly increasing scrap cost and reducing production yield. Combined with practical experience in injection molding workshops, this paper analyzes the inducements of common molding defects one by one, and provides an integrated solution of mold rectification, process adjustment and raw material control to ensure stable mass production of filter shells.

1. Causes and Solutions of Surface Shrinkage Depression

Shrinkage depression is the most common molding problem of water purifier filter shells, which mostly appears at rib positions, the back of screw columns, thickened end faces and wall thickness transition areas. Visible local inner depression on the surface not only reduces the appearance texture, but also destroys the flatness of the sealing ring fitting surface, directly affecting the overall sealing effect. The root cause lies in the excessive local wall thickness and serious glue accumulation of the product. The cooling shrinkage of plastic melt is far greater than the surface solidification amount. In addition, the layout of mold cooling water channels is unreasonable, resulting in slow heat dissipation and uneven shrinkage stress in thick wall areas. Moreover, small gate size, insufficient holding pressure and short holding time lead to insufficient melt feeding and shrinkage compensation. Besides, poor fluidity of raw materials, excessive mixing of recycled materials and inadequate drying will further aggravate shrinkage depression.

The solution requires simultaneous optimization from product structure, mold improvement, injection molding process and raw material management. In product design, the overall wall thickness of the shell is uniformly controlled at 2.0～2.5mm, and material removal and hollowing treatment are carried out for thickened positions such as reinforcing ribs and fixing columns to eliminate hidden dangers of local glue accumulation. In terms of mold, optimize the layout of conformal water channels, encrypt cooling water channels in areas prone to shrinkage, balance the cooling speed of thick and thin walls, and reduce shrinkage difference. In terms of technology, appropriately increase injection pressure and holding pressure, extend the duration of holding pressure compensation and cooling setting time, lower the temperature of the front section of the barrel, and reduce the natural shrinkage rate of melt. At the same time, select high fluidity raw materials with matching grades, strictly control the proportion of recycled materials, fully dry raw materials according to standard before feeding, and avoid shrinkage problems from the source.

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2. Causes and Solutions of Parting Line Flash Burr

Flash burrs mainly appear at the parting surface, thimble holes and insert splicing gaps of filter shell, with excess rubber edges on the product edge, which is easy to scratch hands and jam the mold, and also affects the subsequent assembly positioning accuracy. On the mold side, long-term production leads to wear of parting surface, increased clearance between guide pillar and guide sleeve, misalignment of insert fit, resulting in fine gaps after mold closing. On the process side, insufficient clamping force of injection molding machine, excessive injection speed and high molding pressure make the melt thin at high temperature, which easily penetrates into mold gaps to form overflow flash. In addition, residual rubber scraps and impurities on the parting surface not cleaned will also cause incomplete mold closing and burrs.

Rectification takes mold maintenance and calibration as the core, supplemented by fine adjustment of process parameters. Firstly, calibrate the mold level and concentricity of guide pillar, polish and fit the parting surface, clean rubber dirt and carbon deposits on mold surface and exhaust grooves, grind or replace worn and deformed inserts, and reduce the matching clearance of insert splicing. Appropriately increase the clamping force of injection molding machine to ensure tight mold closing without gaps; lower the injection speed of middle and rear section and holding pressure to avoid high-pressure and high-speed melt impacting mold gaps. At the thin-walled overflow prone position of the shell, add flow resistance groove and hollow structure on the mold to limit melt overflow. Meanwhile, reasonably control barrel and mold temperature to prevent melt from being too thin, and completely eliminate batch flash.
3. Causes and Solutions of Bubble Cavity and Silver Streak DefectsBubble cavities mostly appear in transparent PC filter shells and thick walls of products. Internal hollowness and surface bubbles not only reduce structural strength, but also pose potential safety hazards as water contacting accessories. Silver streaks appear as filament lines on the surface, seriously affecting the appearance qualification rate. The main inducement is that raw materials are not fully dried and damp, and moisture vaporizes at high temperature in the barrel to form bubbles. Excessively fast injection speed causes air entrainment during melt filling. Mold exhaust grooves are blocked or the exhaust position is unreasonably set, so air in the cavity cannot be discharged in time and wrapped inside the product. Excessively high barrel temperature and insufficient screw back pressure lead to free gas generated by thermal decomposition of plastic, which also induces bubbles and silver streaks.

Firstly, strengthen raw material drying control. PP and ABS raw materials are dried at constant temperature for 2 to 4 hours, and the drying time of food-grade PC raw materials is extended to ensure the moisture content meets the standard before feeding. Optimize the mold exhaust system, set reasonable exhaust grooves at melt convergence welding positions and cavity ends, regularly clean carbon deposits and rubber dirt in exhaust grooves to ensure smooth exhaust. Adopt segmented multi-stage injection molding with medium and low speed stable filling to reduce air entrainment; appropriately reduce the temperature of each section of barrel and mold temperature to avoid gas production from high-temperature decomposition of raw materials. Increase screw back pressure and extend screw plasticizing mixing time to discharge air mixed in melt, and eliminate bubbles and silver streaks from the process.

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4. Causes and Solutions of Shell Warping Deformation

Warping deformation of filter shell is manifested as port ellipse, cylinder bending and uneven end face, which makes it impossible to install sealing rings and upper and lower shells normally, and is the main defect causing assembly scrap. The fundamental reason is uneven product wall thickness and inconsistent cooling shrinkage stress. The layout of upper and lower mold cooling water channels is asymmetric, resulting in large difference in cooling speed everywhere. Small demolding draft angle and unreasonable thimble layout cause uneven stress during ejection, resulting in whitening and distortion deformation. In terms of technology, insufficient cooling time, early mold opening and ejection, and improper setting of segmented holding pressure parameters will amplify internal stress and cause permanent deformation.

The optimization scheme adjusts bidirectionally from mold structure and injection molding process. Rearrange the upper and lower mold cooling water channels symmetrically to balance the cooling speed of the shell circumference and end face, and reduce the shrinkage stress difference; reasonably increase the inner wall demolding draft angle, optimize the thimble layout, adopt multi-point balanced ejection to avoid deformation caused by local stress. Appropriately extend the cooling setting time in the process, open the mold and eject after the product is completely solidified, fine-tune the pressure and time of segmented holding pressure to balance internal and external shrinkage stress. Slight deformed products can be corrected by air cooling with shaping fixtures. After batch stabilization, solidify mold parameters and process standards to control the warping defect rate for a long time.
Summary

The molding defects of water purifier filter shells are mainly concentrated in four categories: shrinkage, flash, bubble and warping. All problems are inseparable from four core links: mold structure, cooling and exhaust, process parameters and raw material control. In actual production, it is not allowed to simply adjust the machine and change parameters. We must adhere to mold structure optimization as the foundation, precise debugging of injection molding process as the core, and raw material drying and daily mold maintenance as assistance. Accurately locate problem points for different defects and rectify them item by item. Through systematic management, it can not only completely solve various molding defects, stabilize product appearance, size and sealing performance, but also reduce scrap loss, improve production efficiency, and realize long-term stable mass production of filter shell molds.

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